1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer.
2. Description of Related Art
In an image forming apparatus such as a laser printer, generally, a developer image transferred onto a sheet (e.g., of paper) is thermally fixed thereon in a fixing device. It is known that a sheet subjected to this fixing process tends to curl (curve) due to several factors such as application of mechanical force, evaporation of moisture by heat, and shrinkage of developer. Since the sheet thus curled and then ejected could not be stacked neatly on a sheet output tray, an improved image forming apparatus has been proposed in which after the fixing process a sheet is passed through between conveyor rollers provided in a pair so that a mechanical force is applied to the sheet to reduce an amount of curl thereof.
However, the amount and direction of curl of a sheet may vary with the material and thickness of the sheet, and thus the reduction of the amount of curl could be insufficient or the amount of curl could rather become greater, as the case may be, depending upon the type of the sheet used. With this in view, JP 7-048046 A (see FIG. 1) proposes an image forming apparatus in which a pair of conveyor rollers is swung upward or downward and shifted in position in accordance with an expected amount of curl which may vary with the material and thickness of the sheet so that a substantial reduction of the amount of curl can be ensured.
In the apparatus where a pair of conveyor rollers can be shifted in position, a nip position at which the conveyor rollers nip a sheet is shifted as well, and thus a sheet conveyance path is changed accordingly. When the sheet conveyance path is changed, the sheet would possibly fail to be conveyed smoothly to the nip position of the conveyor rollers, and the sheet could strike a conveyor roller or a guide rib and become jammed at worst.
The present invention has been made in an attempt to eliminate the aforementioned disadvantages in prior art.